1. Field of the Invention
The present invention relates to a linearization of power amplifiers, or more specifically, to linearization of power amplifiers by controlling a predistorter for multilevel quadrature amplitude modulation (QAM) signals via a feedback loop.
2. Discussion of the Prior Art
A major problem with bandwidth efficient QAM schemes is that their performance is strongly dependent on the linearity of the transmission system. The longer the length of the direct propagation channel, the more the need for linearity of the transmission system. A power amplifier employed in such a transmission system is the most non-linear element, and even relatively small distortion components produced by the power amplifier strongly influence the performance of the transmission system.
One prior art solution to the problem of linearization of power amplifiers is the predistortion concept. In this solution, a slowly adaptive predistorter is used to minimize the out-of-band spectral spillage due to power amplifier non-linear distortions. This method monitors the out-of-band power produced by the non-linear amplifier (NLA) and adjusts the predistorter""s parameters to minimize it. More specifically, the predistorter creates the distorted signal from the undistorted modulated signal, and inputs this signal to the NLA. The adaptive feedback path downconverts the bandpass amplifier""s output signal, which is then bandpass filtered to separate the out-of-band signal power from the wanted signal. The out-of-band signal power is then averaged by the power detector and used by the controller to adjust the predistorter""s complex transfer characteristics to minimize the out-of-band signal power. However, to monitor the out-of-band power the prior art solution requires to place an additional receiver at the transmission side of the transmission channel. Thus, this approach has additional costs associated with placing an additional receiver at the transmission side of the channel. The prior art solution also does not take into account any nonlinear channel distortions.
In the modern long loop QAM transmission systems, the need for linearity increases drastically because the length of the transmission is determined by the residual distortions in the direct propagation channel.
What is needed is to monitor the out-of-band signal power by using the existent receiver at the receive side of the direct propagation channel. This would decrease costs and also take into account all distortion, introduced by the NLA and channel itself.
To address the shortcomings of the available art, the present invention provides a method for monitoring the out-of-band signal power by using the receiver at the receive side of the direct propagation channel.
One aspect of the present invention is directed to a method for compensation for nonlinear distortions during propagation of a QAM signal in a direct propagation channel. The direct propagation channel comprises a transmitter/modulator side and a receiver/demodulator side, and the QAM signal preferably includes a constellation of at least 16 phasors.
In the preferred embodiment, the method comprises the following steps: (a) monitoring an out-of-band power produced by a non-linear amplifier (NLA) located at the transmitter/modulator side of the of the direct propagation channel by using a power detector block located at the receiver/demodulator side of the direct propagation channel; (b) generating a nonlinear distortion compensation signal using a controller at the receiver/demodulator side of the direct propagation channel; (c) transmitting the nonlinear distortion compensation signal across the direct propagation channel to an input of a pre-distortion block at the transmitter/modulator side of the direct propagation channel; and (d)adjusting a set of parameters of the pre-distortion block by using the nonlinear distortion compensation signal in order to minimize the out-of-band power produced by the non-linear amplifier (NLA).
In one embodiment, the step (d) of adjusting the set of parameters of the pre-distortion block by using the nonlinear distortion compensation signal further includes the steps of: (d1) deriving a predistortion component from the nonlinear distortion compensation signal at the transmitter/modulator side of the direct propagation channel; and (d2) injecting the predistortion component into the input of the NLA in order to linearize an output signal generated by the transmitter/modulator block. In one embodiment, the step (d1) of deriving the predistortion component from the nonlinear distortion compensation signal at the transmitter/modulator side of the direct propagation channel further includes the steps of: (d1,1) deriving an inphase amplitude predistortion component; and (d1,2) deriving a quadrature amplitude predistortion component. In the alternative embodiment, the step (d1) of deriving the predistortion component from the nonlinear distortion compensation signal at the transmitter/modulator side of the direct propagation channel further includes the steps of: (d1,3) deriving a magnitude predistortion component; and (d1,4) deriving a rotation predistortion component.
In one embodiment, the step of deriving the rotation predistortion component further includes the step of using a plurality of fixed predistortion coefficients based on the NLA backoff power level, wherein the NLA backoff power level ensures that the NLA is in a quasi-linear mode of operation. In the alternative embodiment, the step of deriving the rotation predistortion component further includes the step of using a plurality of predistortion coefficients based on the demodulator. The operation of the NLA is affected by the external parameters including the outside temperature, outside humidity, voltage variations, etc. In one embodiment, the nonlinearity of the NLA caused by these slow changing external parameters can be minimized by adaptively updating the plurality of predistortion coefficients based on the demodulator.
In one embodiment, the step of deriving the predistortion component from the nonlinear distortion compensation signal at the transmitter/modulator side of the direct propagation channel further includes the step of calculating at least two constellation compensation errors for each phasor, wherein a first constellation compensation error is an averaged compensation error, and a second constellation compensation error is a xe2x80x9cselectivexe2x80x9d compensation error for a plurality of xe2x80x9cselectivexe2x80x9d phasors located within the QAM constellation. In alternative embodiment, the step of deriving the predistortion component from the nonlinear distortion compensation signal at the transmitter/modulator side of the direct propagation channel further includes the step of calculating at least two constellation compensation errors for each phasor, wherein a first constellation compensation error is an averaged compensation error, and a second constellation compensation error is an xe2x80x9coutsidexe2x80x9d compensation error for a plurality of xe2x80x9coutsidexe2x80x9d phasors located on a perimeter of the QAM constellation.
In the preferred embodiment, the step of calculating the averaged compensation error, and the xe2x80x9coutsidexe2x80x9d compensation error for each xe2x80x9coutsidexe2x80x9d phasor further includes a number of steps. At first, a xe2x80x9cblindxe2x80x9d mode error (BME) for each xe2x80x9coutsidexe2x80x9d phasor is calculated as a reference error, wherein an QAM phasor is defined as a xe2x80x9cblindxe2x80x9d phasor if it has a maximum peak power without taking into consideration its phase angle. Next, a decision directed equalization (DDE) error for each QAM phasor relative to the a xe2x80x9cblindxe2x80x9d mode reference error (BME) is calculated, wherein the decision directed equalization (DDE) error for the QAM phasor indicates both an amplitude of error and a phase angle for this phasor, and wherein the phase angle for the phasor is indicative of whether the phasor is located inside or outside the xe2x80x9cblindxe2x80x9d power circle. Finally, a plurality of xe2x80x9coutsidexe2x80x9d phasors located outside the xe2x80x9cblindxe2x80x9d power circle is utilized to calculate the predistortion compensation signal.
In the preferred embodiment of the present invention, a reverse feedback service channel is used to transmit the nonlinear distortion compensation signal back to the transmit side of the direct propagation channel.
Another aspect of the present invention is directed to a QAM data transmission system capable of compensation for nonlinear distortions during propagation of a QAM signal in a direct propagation channel. In the preferred embodiment, the system includes: (a) means for monitoring an out-of-band power produced by a non-linear amplifier (NLA); wherein the NLA is located at the transmitter/modulator side of the direct propagation channel; and wherein the means for monitoring is located at the receiver/demodulator side of the direct propagation channel; (b) means for generating a nonlinear distortion compensation signal; wherein the means for generating the nonlinear distortion compensation signal is located at the receiver/demodulator side of the direct propagation channel; (c) means for transmitting the nonlinear distortion compensation signal across the direct propagation channel to an input of a pre-distortion block at the transmitter/modulator side of the direct propagation channel; and (d) means for adjusting a set of parameters of the pre-distortion block by using the nonlinear distortion compensation signal in order to minimize the out-of-band power produced by the non-linear amplifier (NLA).
In the preferred embodiment, means for monitoring the out-of-band power further includes a power detector block located at the receiver/demodulator side of the direct propagation channel, and means for generating the nonlinear distortion compensation signal further includes a controller located at the receiver/demodulator side of the direct propagation channel.